Irritating and annoying acoustic noise and dynamic vibration can be created within an aircraft's cabin due to rotational unbalances and the like of the aircraft's engine(s). For example, on fuselage-mounted aircraft engines, the rotational unbalance(s) cause vibration to be transmitted into the yoke structure, through the intermediate spar structure, and into the aircraft's fuselage. If the vibration of the fuselage is well coupled to the acoustic space within the aircraft's cabin, then annoying, predominantly tonal sound (generally characterized as a low frequency irritating drone) can be generated therewithin. In particular, this drone generally corresponds with the most dominant engine tones, for example, the tones created via the N1 and N2 engine rotations. In aircraft with aft-fuselage-mounted engines, such as the McDonnell Douglas DC-9 aircraft, any rotational unbalance of the engines may result in unwanted and annoying low frequency noise being generated within the aircraft's cabin, and specifically in the aft portion thereof. In general, passengers in the aft portion of the cabin experience this low-frequency tonal noise (drone) related to the N1 and N2 tones of the engine. The N1 and N2 tones are generated by rotational unbalances of the turbine (fan) and compressor stages (compressor) of attached multistage jet engines. Elimination of the N1 and N2 tones can dramatically reduce the discomfort experienced by the passengers, particularly in the aft-most portion of the aircraft's cabin.
Within the prior art, various means have been employed to counter aircraft cabin acoustic noise. These include passive blankets, passive Tuned Vibration Absorbers (TVAs), adaptive TVAs, Active Noise Control (ANC), Active Structural Control (ASC), and Active Isolation Control (AIC). Passive blankets are generally effective in attenuating higher-frequency noise, but are generally ineffective at attenuating low-frequency noise of the type described herein, i.e., low-frequency drone. Furthermore, passive blankets must be massive to reduce low-frequency noise transmission into the cabin. Passive Tuned Vibration Absorbers (TVAs) may be effective at attenuating low-frequency noise, but are generally limited in range and effectiveness. Passive TVAs include a suspended mass which is tuned (along with a stiffness) such that the device exhibits a resonant natural frequency (fn) which generally cancels or absorbs vibration of the vibrating member at the point of attachment thereto. The afore-mentioned disadvantage of passive TVAs is that they are only effective at a particular frequency (fn) or within a very narrow frequency range thereabouts. Therefore, TVAs may be ineffective if the engine frequency is changed and the TVA is not operating at its resonant frequency. Furthermore, passive devices may be unable to generate the proper magnitude and phasing of forces needed for effective vibration suppression and/or control. Passive TVAs are generally attached to the interior stiffening rings or stringers of the fuselage or to the yoke. U.S. Pat. No. 3,490,556 to Bennett, Jr. et al. entitled: "Aircraft Noise Reduction System With Tuned Vibration Absorbers" describes a passive vibration dampening device for use on the pylon of an aircraft for absorbing vibration at the N1 and N2 rotational frequencies.
When a wider range of vibration cancellation is required, various adaptive TVAs may be employed. For example, U.S. Pat. No. 3,487,888 to Adams et al. entitled "Cabin Engine Sound Suppresser" teaches an adaptive TVA where the resonant frequency (fn) can be adaptively adjusted by changing the length of the beam or the rigidity of a resilient cushioning material. Although, the range of vibration attenuation may be increased with adaptive TVAs, they still may be ineffective for certain applications, in that their range of adjustment may not be large enough or they may not be able to generate enough dynamic forces to adequately reduce acoustic noise or vibration experienced within the aircraft's cabin.
In some applications where a higher level of noise attenuation is desired, Active Isolation Control (AIC) systems provide another means for controlling noise within an aircraft's cabin. In Prior Art FIG. 1, an aircraft with multiple aft-fuselage-mounted turbofan engines is shown. AIC systems include active mountings, such as 12a, 12b, 12c, and 12d, which include an actively driven element contained therein, to provide the active control forces for isolating vibration and preventing its transmission from the engines 18L and 18R into the pylon structures 28L and 28R. The resultant effect is preferably a reduction of annoying interior acoustic noise in the aircraft's cabin 44. Known AIC systems include the feedforward type, in that reference signals, such as from reference accelerometers 49L and 49R, are used to provide a reference signal indicative of the N1 and N2 vibrations of engines, 18L and 18R. Error sensors, such as a plurality of microphones 42, provide error signals indicative of the residual noise at various locations in the aircraft cabin 44. Specifically, in known AIC systems, active mountings, such as 12a-d are attached between an aircraft yoke 32L and 32R and the aircraft's engine 18L and 18R. The reference signals and error microphones 42 are processed by a digital controller 46 to generate drive signals of the appropriate phase and magnitude (anti-vibration) to reduce vibration transmission from the engine to the yoke, and resultantly controlling and/or reducing the interior acoustic noise.
Copending U.S. patent application Ser. No. 08/260,945 entitled "Active Mounts For Aircraft Engines" describes several AIC systems. Furthermore, commonly assigned U.S. Pat. No. 5,174,552 to Hodgson et al. entitled "Fluid Mount With Active Vibration Control" describes the details of one type of active fluid mounting. It should be understood, that in some applications, there may be insufficient space envelope to incorporate the active element within the active mounting. Furthermore, there may be alternate vibration paths into the structure or the appropriate actuation directions required for vibration attenuation may be difficult to accomplish within the environment of an active mounting. Furthermore, modification to the mounting system, to incorporate active elements may reduce the amount of load bearing surface, possibly reducing the drift-life expectancy of the mounting system.
Active Noise Control (ANC) systems are also well known. As described with reference to Prior Art FIG. 2, ANC systems may be used on turboprop aircraft or the like, and include a plurality of acoustic output transducers, such as loudspeakers 16a, 16b, 16c, and 16d, strategically located within the aircraft's cabin 44 and attached to the aircraft's trim. These loudspeakers are driven responsive to input signals from input sensors and error signals from error sensors 42 disbursed within the aircraft's cabin 44. Input signals may be derived from engine tachometers, accelerometers, or the like, which are placed on the engines 18L and 18R, or reference sensors 14L and 14R located on the fuselage in the area of the aerodynamic propeller wash generated by the propellers 17L and 17R driven by engines 18L and 18R mounted on wings 15L and 15R. The output signals to the loudspeakers 16a-16d, in ANC systems are generally adaptively controlled via a digital controller 46 according to a known feedforward type adaptive control algorithm, such as the Filtered-x Least Mean Square (LMS) algorithm, or the like. Copending U.S. patent application Ser. No. 08/553,227 to Billoud entitled "Active Noise Control System For Closed Spaces Such As Aircraft Cabins" describes one such ANC system. ANC systems have the disadvantage that they do not generally address any mechanical vibration problems and may be difficult to retrofit to existing aircraft. Furthermore, as the frequency of noise increases, large numbers of error sensors and speakers are required to achieve sufficient global noise attenuation.
Certain ASC systems, known in the prior art, may solve this problem of needing a large number of error sensors by attacking the vibrational modes of the aircraft's fuselage directly. For example, by attaching "a vibrating device such as an actuator or a shaker which is directly connected to the interior surface of the fuselage in order to introduce a vibration directly into the fuselage surface", as described in U.S. Pat. No. 4,715,559 to Fuller, global attenuation can be achieved with a minimal number of error sensors. However, the modifications necessary to retrofit AVAs in this manner may be prohibitive, as the interior trim may have to be removed and structural modifications made have to be made to the stringers or stiffening-ring frames. Furthermore, for control of N2 tones, an exceedingly large number of AVAs may be needed. Therefore, prior art ASC systems are necessarily difficult to retrofit and may require the use of many shaker/actuators to effectuate control of higher-order tones. U.S. Pat. No. 5,310,137 to Yoerkie, Jr. et al. describes the use of AVAs to cancel high-frequency vibrations of a helicopter transmission. Notably, Yoerkie, Jr. et al. is a feedback-type system.
Further descriptions of AVAs and active mounts can be found in Copending U.S. application Ser. No. 08/322,123 entitled "Active Tuned Vibration Absorber" and copending PCT application PCT/US95/13610 entitled "Active Systems and Devices Including Active Vibration Absorbers (AVAs)."
Therefore, there is a recognized need for an ASC system which provides active attenuation to effectively minimize vibration within the structure attached between the engine and the fuselage of an aircraft with the result of reducing annoying acoustic noise and mechanical vibration within the aircraft's cabin throughout its entire frequency range, and without the need for major modification of the fuselage or the aircraft engine mountings, thus allowing ease of retrofit of the system.